The present invention relates generally to refrigeration systems and more particularly to means for controlling the oil levels in two-stage serially connected compressors.
A well-known class or type of compressor such as are used for example in refrigeration and air-conditioning system is a hermetically sealed compressor which is characterized by the fact that the compressor and its electric drive motor are enclosed within a single housing and the drive shaft of the motor is connected to the crankshaft of the compressor.
There are many refrigeration applications requiring cooling to a very low temperature. Some applications may call for the installation of two or more compressors in series or parallel. As the refrigerant passes through a multi-stage compressor system, a certain amount of compressor lubricating oil becomes intermixed with refrigerant. This oil migration through the system sometimes prevents full lubrication of the compressors at all times. Oil leakage out of the compressors may become severe enough for the compressors to be starved of lubricating oil and fail.
A primary difficulty in serially connected compressors is of maintaining an adequate oil level in the crankcases of the different compressors which may receive recirculating or migrating oil, and lose oil again at varying rates. The rate at which lubricating oil is discharged out of a compressor along with compressed refrigerant is called the oil loss rate or simply the loss rate.
In the prior art, oil level controls operated by means of conduits, between the various compressors, connected with pumps, valves, oil holding tanks or other means for equalizing the oil level between the compressors.
A prior art patent issued to Miner, U.S. Pat. No. 3,360,958, discloses a lubrication apparatus disposed within a multiple compressor system having an oil accumulator within the refrigerant line that traps circulating oil. A float valve permits captured oil to travel into the compressor oil sumps. The oil sumps are connected together with conduits located at the desired oil levels within each compressor. Excess oil collects into a receiver where refrigerant flow pulls oil back into the refrigerant stream through an ejector.
Problems with prior art designs include a varying flow of oil within the lubricant systems, dependant upon refrigerant flow and extra lubrication system parts that may wear out and leak.
The present invention is directed to overcoming the aforementioned problem associated with compressors connected in series where it is desired to provide adequate oil levels within the compressor housings with a substantial decrease in the number of parts.